The PTEN tumor suppressor gene plays a critical role in a number of human cancers, including prostate. PTEN encodes a lipid phosphatase that plays a key role in regulating the PI3-kinase/Akt signaling pathway, providing a compelling explanation of why loss of PTEN function leads to cancer. Our group has shown that: (i) PTEN function is lost in up to 50 percent of advanced prostate cancers; (ii) PTEN regulates the PI3- kinase/Akt pathway in prostate cancer cells; (iii) PTEN binds a multi-PDZ scaffold protein (MAGI-2) in prostate and brain cells and (iv) MAGI-2 can regulate the phosphatase activity of PTEN. Here we propose to characterize the regulation of PTEN by MAGI-2 in detail by examining the effect of MAGI-2 on the tumor suppressor activity of PTEN and by examining the importance of the MAGI-2/PTEN complex in maintaining the integrity of signaling through the PI3- kinase/Akt pathway. We will also examine the possibility that MAGI-2 itself may play a role in oncogenesis through studies of its expression profile in prostate and brain cancers. Finally, we will study the importance of the MAGI-2/PTEN interaction in the context of a whole animal by creating targeted deletions of the MAGI-2 binding domain in the C-terminus of PTEN and by creating a MAGI-2 knockout mouse. Together, these experiments will define a novel mode of PTEN regulation in human cancers.